Membrane biofouling and tissue changes in the foreign body response are known to cause detrimental reductions of analyte transport into implanted biosensors. The relative contribution of each phenomenon is unknown. Hollow fiber microdialysis probes were employed to assess the effect of subcutaneous implantation on glucose flux through polymeric membranes in rats over 8 days and to differentiate the transport effects of biofouling versus tissue changes. Three commercially available membranes were examined: poly(ether sulfone) (PES), polyacrylonitrile (PAN), and polycarbonate (PC). As measured by glucose recovery (the ratio of microdialysis glucose to blood glucose concentrations), transport through PES membranes was significantly less on day 2 than day 0 (39% decrease, p < 0.05) whereas PAN and PC showed no significant decreases in flux until day 8 (42 and 43%, respectively). Application of a transport model to glucose recovery data obtained before implantation in vivo and after explantation indicated that mass transport resistances originating from biofouling and tissue compartments increased between days 0 and 8. However, on average the biofouling layer adherent to the probe created substantially less resistance to glucose transport (12-24% of total) than did the tissue that surrounded the probe. These results suggested that future material developments for biosensors should be directed at understanding and modifying transport properties of tissues at the implant site.

The mucosal immune system fulfils the primary function of defence against potential pathogens that may enter across vulnerable surface epithelia. However, a secondary function of the intestinal immune system is to discriminate between pathogen-associated and ‘harmless’ antigens, expressing active responses against the former and tolerance to the latter. Control of immune responses appears to be an active process, involving local generation of IgA and of regulatory and/or regulated T lymphocytes. Two important periods of maximum exposure to novel antigens occur in the young animal, immediately after birth and at weaning. In both cases the antigenic composition of the intestinal contents can shift suddenly, as a result of a novel diet and of colonisation by novel strains and species of bacteria. Changes in lifestyles of man, and husbandry of animals, have resulted in weaning becoming much more abrupt than previously in evolution, increasing the number of antigens that must be simultaneously evaluated by neonates. Thus, birth and weaning are likely to represent hazard and critical control points in the development of appropriate responses to pathogens and harmless dietary and commensal antigens. Neonates are born with relatively undeveloped mucosal immune systems. At birth this factor may prevent both expression of active immune responses and development of tolerance. However, colonisation by intestinal flora expands the mucosal immune system in antigen-specific and non-specific ways. At weaning antibody to fed proteins can be detected, indicating active immune responses to fed proteins. It is proposed that under normal conditions the ability of the mucosal immune system to mount active responses to foreign antigens develops simultaneously with the ability to control and regulate such responses. Problems arise when one or other arm of the immune system develops inappropriately, resulting in inappropriate effector responses to harmless food proteins (allergy) or inadequate responses to pathogens (disease susceptibility).

A comprehensive study of how age and weaning affect intestinal structure and enterocyte ability to digest and absorb nutrients has been carried out in 4-and 6-weekold piglets.Villus length, which did not change significantly in unweaned piglets 4-6 weeks after birth, was halved 5 days after weaning. Crypt depth, which increased normally in unweaned piglets, is further increased by weaning in both 4-and 6-week-old animals.Lactase activity, which decreased normally with age, was inhibited more than a-glucosidase by weaning. Weaning of 6-week-old piglets also caused a significant increase in maltase II and III activities. Alkaline phosphatase activity was unaffected by age or weaning in 4-and 6-week-old piglet intestine.Na-dependent alanine transport was reduced in 6-compared with 4-week-old unweaned piglet intestine. Weaning inhibited Na-dependent alanine transport in 4-but not 6-week-old pigs. Na-independent alanine transport, which was considerably less than that found in the presence of Na, was not noticeably affected by age or weaning.Weaning-induced problems in intestinal function appear from the present results to be caused more by changes in intestinal structure and specific loss of digestive enzymes rather than by any gross change in absorptive function. The possible role of immune as well as nutritional factors in causing these weaning-dependent changes in intestinal function is discussed.

The delivery of certain living microorganisms in food has long been suggested as having positive health effects in humans. This practice has extended into food animal production, with a variety of microorganisms being used; lactic acid bacteria, variousBacillusspecies and the yeastSaccharomyces cerevisiaehave been particularly used in the pig industry. The increased interest in probiotics is essentially due to the problem of microbial resistance to antibiotics and following the ban of the use of antibiotics in animal production, probiotics being considered an alternative means to reduce pathogen infection and improve animal health especially around the time of weaning. However, there is still a need to clarify the probiotic effectiveness in pigs, and the underlying mechanisms. When assessing the efficacy of probiotics one must consider the particular strain of organism being used and the production stage of the pigs being treated. The reproducible delivery of probiotics in industrial pig production is problematic as maintenance of viability is key to their beneficial activity, but difficult to achieve with commonly used feed processing technologies. One specific context where probiotics organisms may be reliably delivered is in systems utilising fermented liquid feeds. Liquid feed may be fermented by the activity of wild lactic acid bacteria or may be stimulated using specific isolates as ‘starters’; the latter system has advantages in terms of reproducibility and speed of fermentation. The farm context in which the organism is used is likely to be critical; the use of probiotics is more likely to result in measurable economic gains in animals living in sub-optimal conditions rather than in those reared in the highest welfare and husbandry conditions. The establishment of a beneficial lactic acid bacteria population at birth may lead to healthier animals, this may be most effectively achieved by treating sows, which provide an amplification step and flood the neonatal pigs’ environment with desirable bacterial strains. In contrast, it may be sufficient to provide a supportive, protective microbiota around the time of weaning as this is a time of major crisis with instability and loss of certain bacterial populations.

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